Picture display arrangement provided with a display tube

ABSTRACT

In a TV display system, a circuit for regulating the voltage at the output terminals of the high voltage supply for the CRT acceleration anode. The circuit includes a VDR and a resistor in series circuit across said output terminals and a voltage divider network connected across a source of DC voltage. A diode is connected between a point on the series circuit and a point on the voltage divider with a polarity and in a manner such that the diode is cut off at the nominal operating voltage at said terminals and is rendered conductive when the voltage increases above a given value. The beam intensity electrode of the CRT is connected to a point on the voltage divider such that conduction of the diode increases the electrode voltage to increase the electron beam intensity thereby increasing the loading effect on the high voltage supply.

Enited States P PICTURE DISPLAY ARRANGEMENT PROVIDED WITH A DISPLAY TUBE 5 Claims, 1 Drawing Fig.

L'.S.Cl 178/75, 4 315/30, 323/22, 328/267 Int. Cl H04n 5/44 Field of Search 178/6. 7.3,

I56] References Cited UNITED STATES PATENTS 3.375.436 3/1968 Demon l78/7.5 E

Primary E.raminer-Robert L. Richardson Attorney-Frank R Trifari ABSTRACT: In a TV display system, a circuit for regulating the voltage at the output terminals of the high voltage supply for the CRT acceleration anode. The circuit includes a VDR and a resistor in series circuit across said output terminals and a voltage divider network connected across a source of DC voltage. A diode is connected between a point on the series circuit and a point on the voltage divider with a polarity and in a manner such that the diode is cut off at the nominal operating voltage at said terminals and is rendered conductive when the voltage increases above a given value. The beam intensity electrode ofthe CRT is connected to a point on the voltage divider such that conduction of the diode increases the elec- 7.5; 328/9. I0. 259. 267'. BIS/30; 323/19, 22; trode voltage to increase the electron beam intensity thereby 338/20 increasing the loading effect on the high voltage supply.

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PATENTEU JUH1 5 I97! INVENTOR.

ANTON/US BoEKmRs'T PICTURE DISPLAY ARRANGEMENT PROVIDED WITH A DISPLAY TUBE This invention relates to a picture display arrangement provided with a display tube and a high-voltage generator between the output terminals of which a high DC voltage is generated for the display tube and between which terminals a series arrangement of an element having a nonlinear currentvoltage characteristic and a resistor is provided, the arrange ment also including a potentiometer which applies a low DC voltage to a tube electrode that controls the intensity of the electron beam in the display tube.

By means of such a display arrangement, electric signals can be converted into an optical picture obtained by means of fluorescence on the display screen of the display tube. The high DC voltage generated by the high-voltage generator is applied to an anode of the display tube for accelerating the electrons in said display tube. In an embodiment wherein the generator is designed as a flyback driven high-voltage genera tor, a sawtooth deflection current is supplied which can deflect the electron beam with the aid of deflection coils surrounding the display tube. The intensity of the electron beam provided by an electron gun is dynamically controlled on the one hand by the electrical signal to be displayed and on the other hand is statically adjusted with the aid of the said potentiometer. To this end the electrical signal to be displayed is applied between the cathode and a first electrode, the so-called wehnelt cylinder, while the first electrode can be used or a second electrode is provided for the brightness control which is adjustable by means ofthe potentiometer.

The variations ofthe intensity in the electron beam required for the display of the optical picture on the display screen have the result that the high-voltage load also varies. Since a uniform relation between the high-voltage load and the deflection current is required to produce the correct picture dimensions of the screen, a display arrangement is often provided with a control circuit for obtaining the desired ratio. The signal required for the control circuit, which is a measure of the high voltage, may be derived in known manner from the series arrangement of the element having a nonlinear current voltage characteristic and a resistor.

The high DC voltage required for a picture display arrangement has a value which, dependent on the field of application, may be 18 kv for, for example, monochrome television, 25 kv for color television and 60 kv and more for projection television. For a certain arrangement it applies that the high DC voltage must not increase to a much higher value than the nominal value. Flashovers with all the destructive results and risk of fire attended therewith could then occur both in the generator and in the display tube. Such high voltage values may also give rise to generation of X-ray radiation in the arrangement. The X-ray radiation, which is dangerous for living beings, must be reduced to a safe level by taking protective steps. A safe level can be obtained both by means of a construction surrounding the place of generation and absorbing X-ray radiation and by limiting the maximum value to which the high DC voltage may increase with the aid of a safety cir' cuit. For the safety circuit the requirement applies that it operates under all circumstances so that a simple circuit containing as few elements as possible is preferredv lt is also desirable that the user of the display arrangement receives an unambiguous indication that the safety circuit has started to operate so that the use of the arrangement can be discontinued.

An object of the invention is to provide a high-voltage safety circuit which is simple and reliable in operation and gives an indication when it starts to operate. To this end the picture display arrangement according to the invention is characterized in that the arrangement is provided with a high-voltage safety circuit comprising an element conducting current in one direction. This element is provided between a tapping on the series arrangement connected to the output terminals of the generator and a terminal of the potentiometer across which a more or less constant voltage is applied, said unidirectional conducting element becoming conducting when the high DC voltage exceeds a maximum permitted value which is related to the value of the said more or less constant voltage. The unidirectional element applies a DC voltage to said tube electrode through the potentiometer thereby in creasing the intensity ofthe electron beam.

The invention is based on the insight that a safe device without risk of fire and without dangerous X-ray radiation is obtained by exerting an influence on the brightness control through a diode, for example, for values of the high DC voltage which are higher than the maximum permitted values. In fact, by increasing the intensity of the electron beam the high DC voltage will be loaded more strongly on the one hand and thus will decrease in value, while on the other hand a pale picture is produced which is poor in contrast because the otherwise dark parts in the picture also become bright and additionally because the section of the electron beam has increased. Instead' of being used partly for generating X-ray radiation, the high-voltage power is dissipated for a greater part in the display tube.

In order that the invention may be readily carried into effect, it will now be described in detail, by way ofexample, with reference to the accompanying diagrammatic drawing showing one embodiment of a display arrangement according to the invention.

The FIGURE shows a high-voltage generator 1 in which a high DC voltage V is generated between an output terminal connected to ground and an output terminal 2 under the influence ofa signal (not shown) applied to an input terminal 3, Generator 1 may be designed as a pulse high-voltage generator for generating the voltage V,, only, and as a flyback driven high-voltage generator in which a sawtooth deflection current is also generated.

The voltage V,, occurring at the output terminal 2 is applied to a series arrangement consisting of an element 4 having a nonlinear current-voltage characteristic such as, for example, a voltage-dependent resistor, and a resistor 5 the other end of which is connected to ground. Resistor 5 may be designed as a potentiometer having a tapping which is connected to a control circuit 6. Control circuit 6 may serve to obtain a uniform relation between the value of the high DC voltage V,, and the amplitude ofa sawtooth deflection current which is further regarded as irrelevant here. lt is alternatively possible for control circuit 6 to keep the value of the voltage V,, more or less constant despite a varying load on said voltage.

An anode 8 of a picture display tube 7 is connected to output terminal 2. Display tube 7 is indicated by a diagrammatical section further showing: a cathode 9 a first electrode 10 which may be designed in known manner as a wehnelt cylinder; a second electrode 11 by means of which the intensity of the electron beam provided by cathode 9 also can be adjusted; and a third electrode 12 which serves for focusing purposes. For generating an electron beam, not shown, the electrical signal to be displayed on the screen 13 is applied between the terminals connected to cathode 9 and electrode 10. This signal is the video signal for television purposes. For monochrome television the electrode 10 may, for example, be connected to ground while for color television the brightness signal is often applied to the cathode 9 and the color dif ference signal to the electrode 10. A multiple design of cathode 9 and electrodes 10, 11 and 12 is of course possible, dependent on the design of tube 7 and/or the choice of the color television system used. A threefold design is, for example, used in a tube 7 designed with a so-called shadow mask.

An electrode 11, which in the embodiment shown in the FIGURE serves for adjustment of the brightness of the picture on the screen 13, is connected through a resistor 14 to the tapping of a resistor 15 designed as a potentiometer, one end of which is connected through a resistor 16 to ground and the other end of which is connected through a resistor 17 to a terminal to which a voltage V, is applied. The terminal carrying the voltage V, may form part of a voltage source V (not shown) of which a terminal carrying the negative voltage V, is considered to be connected to ground. Resistors 15 through 17 form an adjusting potentiometer 18 by which the value of the voltage applied to electrode 11 and hence the intensity of the electron beam can be adjusted arbitrarily. Embodiments of the potentiometer 18 other than the one shown are also possible, it will also be evident that electrode instead of electrode 11 can also be used for the brightness control, all this being indicated by a broken line.

Electrode 12, which serves to focus the electron beam, may be connected to a tapping on the element 4 having a nonlinear current-voltage characteristic so that in the event of a varying value of voltage V, at output terminal 2, the voltage applied to electrode 12 also varies, as in desirable for a correct focusing.

Other components of the display arrangement which are required for the operation thereof, for example, the deflection coils for display tube 1, are irrelevant for an understanding of the present invention and therefore have been omitted.

The arrangement described so far functions satisfactorily under normal operating conditions. The control circuit 6 determining the value of the high DC voltage V,, controls the said value in such manner that the acceleration provided to the electron beam in tube 7 by the anode 8 and a sawtooth current generated in the deflection coils produces a picture of the correct dimensions on screen 13. If, however, the value of the DC voltage V, increases for some reason or other, for example, due to a failure of control circuit 6, the dangerous phenomena already described may occur. In fact, the electron beam in tube 7 may be accelerated so strongly that, for example, X-ray radiation emerges from the screen. For monochrome television the cause resides in the fact that the inner side of the screen 13 is provided with a thin light-reflecting aluminum coating so as to increase the luminous efficiency of the display tube 7. For color television, in which tube 7 is designed, for example, as a shadow-mask tube, the metal shadow mask is the cause of the X-ray radiation. If voltage V,, has too high a value it is also possible that flashovers occur between anode 8 and other tube components, for example, the electrodes 10, 11 or 12, if the electron beam in tube 7 is substantially cut off because the video signal has dropped out or represents black. The result is that an anode 8 provided with metal parts may emit X-rays,

To prevent the voltage V,, from increasing to inadmissible values, the high-voltage generator according to the invention is provided with an element conducting current in one direction, which element is designed as a diode l9 and which is provided between a tapping 20 of the series arrangement (4,5) and a terminal 21 of potentiometer 18. By providing one signal diode 19 a simple high-voltage safety circuit is obtained in this manner which is reliable in operation.

When the arrangement operates normally, the diode 19 will not conduct since the value of the voltage across tapping 20 is lower than that across terminal 21. If, however, the value of the voltage V, increases by, for example, 10 percent for some reason or other, for example, by failure of control circuit 6, the current flowing in the series arrangement (4,5) will increase by, for example, 50 percent under the influence of the nonlinear current-voltage characteristic of element 4. The result is that the value of the voltage across tapping 20 also increases by 50 percent and becomes, for example, equal to that across terminal 21 so that diode 19 becomes conducting. Diode 19 will continue to conduct for a possible further increase of the value of voltage V,,. The conducting of diode 19 has the result that a more positive voltage is impressed on terminal 21 and hence on electrode 11. The intensity ofthe electron beam in tube 7 and hence the load of on the voltage V,, will increase so that this voltage will increase to a lesser degree than would be the case without diode 19. The increased intensity and hence greater section of the electron beam in tube 7 will be the reason why the picture on the screen 13 will become pale and poor in contrast. The user of the arrangement thus receives an indication about the interference and may discontinue the use of the arrangement.

The voltage across terminal 21 of potentiometer l fserves to obtain a substantially unifonn maximum pennissible value of voltage V, which is to be more or less independent of the position of the tapping of the resistor 15 designed as a potentiometer. This can be obtained in a simple manner by choosing the value of resistor 15 to be small relative to that of resistor 16 and/or by rendering the current flowing through resistor 14 small relative to that flowing through resistor 16.

It will be evident that the voltage for control circuit 6 which is derived from the voltage V, need not absolutely derived from the series arrangement (4,5), but series arrangement (4,5) may also be used for the safety circuit only. Tapping 20 may alternatively form part of resistor 5 so that only a part of the voltage across resistor 5 is applied to the anode of diode 19.

It will also be evident that ii makes very little difference to the present invention whether the increase of intensity of the electron beam upon too high an increase of voltage V, is caused by the fact that the resistor 14 is connected to electrode ll of the display tube 7 or to the electrode 10.

lclaim:

l. A picture display arrangement comprising a cathode-ray display tube, a high-voltage generator having output terminals between which a high DC voltage is generated for the display tube, means connecting a series arrangement of an element having a nonlinear current-voltage characteristic and a resistor between said terminals, a voltage divider connected across a DC source of relatively constant voltage, means connecting an electrode of the display tube that controls the intensity of the electron beam to a point on said voltage divider at which a low DC voltage appears, and a high-voltage safety circuit comprising a unidirectional current conducting element connected between a tapping on the series arrangement and a terminal of the voltage divider across which a more or less constant voltage appears and in a manner such that the unidirectional element is rendered conducting when the high DC voltage exceeds a given maximum value which is related to the value of said more or less constant voltage, said unidirectional element thereby applying a DC voltage to said tube electrode via the voltage divider for increasing the intensity of the electron beam.

2. An arrangement as claimed in claim 1 wherein said nonlinear element comprises a voltage dependent resistor and said unidirectional element comprises a diode with its cathode connected to said terminal of the voltage divider.

3. An arrangement as claimed in claim 2 wherein said highvoltage generator comprises the horizontal deflection circuit of the picture display arrangement, and a control circuit having an input terminal connected to a point on said series arrangement and an output terminal coupled to said horizontal deflection circuit for applying a control voltage thereto by means of which said deflection circuit regulates the value of the high DC voltage at said output terminals.

4. A cathode-ray tube display system comprising, a source of high DC voltage for supplying a nominal operating voltage at a pair of output terminals, means connecting the accelerating anode of said tube across said terminals, a first voltage divider network comprising an element having a nonlinear current-voltage characteristic and a resistor connected in series across said output terminals, a second source of DC voltage, a second voltage divider network connected across said second voltage source, a unidirectional current element connected between a point on said first voltage divider network and a point on said second voltage divider network with a polarity and in a manner such that the unidirectional element is nonconductive at said nominal operating voltage and is rendered conductive upon a preselected increase of said high DC voltage above the nominal voltage, and means connecting a beam intensity electrode of said tube to a point on said second voltage divider network such that conduction of said unidirectional element increases the DC voltage applied to said electrode thereby increasing the election beam intensity of the cathode-ray tube.

5. A display system as claimedin claim 4 wherein said nonlinear element comprises a voltage dependent resistor and said second voltage divider network includes a potentiometer having an arm connected to said beam intensity electrode. 

1. A picture display arrangement comprising a cathode-ray display tube, a high-voltage generator having output terminals between which a high DC voltage is generated for the display tube, means connecting a series arrangement of an element having a nonlinear current-voltage characteristic and a resistor between said terminals, a voltage divider connected across a DC source of relatively constant voltage, means coNnecting an electrode of the display tube that controls the intensity of the electron beam to a point on said voltage divider at which a low DC voltage appears, and a high-voltage safety circuit comprising a unidirectional current conducting element connected between a tapping on the series arrangement and a terminal of the voltage divider across which a more or less constant voltage appears and in a manner such that the unidirectional element is rendered conducting when the high DC voltage exceeds a given maximum value which is related to the value of said more or less constant voltage, said unidirectional element thereby applying a DC voltage to said tube electrode via the voltage divider for increasing the intensity of the electron beam.
 2. An arrangement as claimed in claim 1 wherein said nonlinear element comprises a voltage dependent resistor and said unidirectional element comprises a diode with its cathode connected to said terminal of the voltage divider.
 3. An arrangement as claimed in claim 2 wherein said high-voltage generator comprises the horizontal deflection circuit of the picture display arrangement, and a control circuit having an input terminal connected to a point on said series arrangement and an output terminal coupled to said horizontal deflection circuit for applying a control voltage thereto by means of which said deflection circuit regulates the value of the high DC voltage at said output terminals.
 4. A cathode-ray tube display system comprising, a source of high DC voltage for supplying a nominal operating voltage at a pair of output terminals, means connecting the accelerating anode of said tube across said terminals, a first voltage divider network comprising an element having a nonlinear current-voltage characteristic and a resistor connected in series across said output terminals, a second source of DC voltage, a second voltage divider network connected across said second voltage source, a unidirectional current element connected between a point on said first voltage divider network and a point on said second voltage divider network with a polarity and in a manner such that the unidirectional element is nonconductive at said nominal operating voltage and is rendered conductive upon a preselected increase of said high DC voltage above the nominal voltage, and means connecting a beam intensity electrode of said tube to a point on said second voltage divider network such that conduction of said unidirectional element increases the DC voltage applied to said electrode thereby increasing the election beam intensity of the cathode-ray tube.
 5. A display system as claimed in claim 4 wherein said nonlinear element comprises a voltage dependent resistor and said second voltage divider network includes a potentiometer having an arm connected to said beam intensity electrode. 